Poly-2-pyrrolidone (also known as nylon-4) is composed of repeating C.sub.4 structural units -- NH(CH.sub.2).sub.3 C(O)----. The solid, melt-spinnable homopolymer of 2-pyrrolidone is produced by the alkaline-catalyzed polymerization of 2-pyrrolidone in the presence of carbon dioxide, using a catalyst composed of alkali metal lactamate or tetraalkyl ammonium lactamate (see U.S. Pat. No. 3,721,652). Copolymers of 2-pyrrolidone with C.sub.7 -C.sub.12 lactams have C.sub.7 -C.sub.12 structural units substituted for some of the C.sub.4 structural units of poly-2-pyrrolidone. Copolymers of 2-pyrrolidone with capryllactam (C.sub.8) or laurolactam (C.sub.12) reportedly have been made by alkaline-catalyzed polymerization, Czech Pat. No. 158,341, Chem. Ab. 84: 44962d.
It is an object of the present invention to achieve the rapid alkaline-catalyzed copolymerization of 2-pyrrolidone with C.sub.7 -C.sub.12 lactams in the presence of carbon dioxide. It is a further object of the present invention to produce a solid, melt-spinnable copolymer of 2-pyrrolidone and C.sub.7 -C.sub.12 lactams which contains more than 10 mol percent of C.sub.7 -C.sub.12 structural units. It is a further object of the present invention to produce a solid copolymer of 2-pyrrolidone and a C.sub.7 -C.sub.12 lactam possessing even greater thermal stability than the solid 2-pyrrolidone homopolymer.